Advice on a place to camp near Mt. Rainier and Mt St Helens

Provided you're willing to stay in an RV park (you mentioned "campground") there is a very nice one that is convenient to both mountains. "Shady Furs" located in Randal which is about half way between the two. VERY nice folks operating the park (they brought us fresh produce to our door every couple of days). Peaceful setting, in a rural community with restaurant and stores close at hand.